The Sun Wasn't Warm Enough for Life When Life Began

Contributing Editor

You don’t have to be a creationist to know there are plenty of questions about the origin of life remaining to be answered. Even when an answer seems to be coming together, new evidence can undermine it, rule out a possible solution, and cloud what appeared to be clearing. That happened this week with the faint young Sun paradox, and it’s made the question of “how did life happen on Earth?” that much more daunting.

The faint young Sun paradox is this: Stars get warmer as they age, and the Sun is no exception. It used to be cooler, it’s getting hotter, and eventually the Sun is going to boil away our oceans.

Between 3.8 and 2.4 billion years ago, the Archean eon, life evolved on Earth. At the same time the Sun was only running at about 70 percent of what it does now. At that rate, the Earth shouldn’t have had any liquid water, and consequently should not have had life. However, that’s not what the record shows.

“The Earth should have been permanently glaciated but geological evidence suggests there were no global glaciations before the end of the Archean and that liquid water was widespread," said Ray Burgess, from the University of Manchester. This is the paradox of the faint young Sun.

So how was the Earth wet wild and full of life, when the Sun was just a cool young star? Some believe that solar wind was stronger. Up until this last week, researchers believed that it was possible that greenhouse gases kept the Earth sufficiently warm enough to keep that primordial ooze flowing. But researchers from the University of Manchester and the Institut de Physique du Globe de Paris demonstrated, in a paper published in Science, why this wasn’t the case.

"To counter the effect of the weaker Sun, carbon dioxide concentrations in the Earth's atmosphere would need to have been 1,000 times higher than present," said lead author Bernard Marty, from the CRPG-CNRS University of Lorraine. "However, ancient fossil soils—the best indicators of ancient carbon dioxide levels in the atmosphere—suggest only modest levels during the Archean. Other atmospheric greenhouse gases were also present, in particular ammonia and methane, but these gases are fragile and easily destroyed by ultraviolet solar radiation, so are unlikely to have had any effect."

But carbon dioxide isn’t the only greenhouse gas. Marty and his team looked at old and “exceptionally well-preserved rocks,” from northern Australia to test for argon and nitrogen. If nitrogen levels were high enough it would amplify carbon dioxide’s effect. But there wasn’t enough nitrogen in the Archean atmosphere for this to work either.

Their deductions from the argon and nitrogen tests did allow for higher levels of carbon dioxide that were “at odds with the estimates based on fossil soils,” according to Burgess, “which could be high enough to counteract the effects of the faint young Sun and will require further investigation."

So we don’t know as much as we thought we did, and we may not even know as much as we need to know how little that we know. Science marches on, pursuing how it is that, to paraphrase a great scientist, life found a way.